The original development and subsequent application in this laboratory of a unique system of primary in vitro stimulation of human peripheral blood B lymphocytes with polyclonal activators or specific antigens and subsequent measurement of single cell antibody responses by a hemolysis-in-gel plaque forming cell assay has allowed us to probe many of the complex events and cellular interactions involved in human B cell function. This approach has been employed in the delineation of cell triggering and regulatory mechanisms in normal immune responses as well as in a group of diseases of aberrant immunological reactivity. We have demonstrated the presence of distinct subpopulations of helper and suppressor cells of normal B cell responses as well as aberrancies of these regulators of B cell function in hypersensitivity disease states. In addition, the production of soluble regulatory factors of human B cell triggering have been induced in vitro and have been shown to play a significant role in modulation of B cell function. The differential sensitivity of various stages of B cell activation, proliferation and differentiation to regulation by corticosteroids has been demonstrated. This has clearcut relevance in understanding the mechanisms of regulation of B cell function as well as the mechanisms of action of corticosteroids in immunologically mediated diseases. BIBLIOGRAPHIC REFERENCES: Hunninghake, G. W. and Fuaci, A. S.: Immunological reactivity of the lung. IV. Effect of cyclophosphamide on aveolar macrophage cytotoxic effector function. Clin. Exp. Immunol. 27: 555-559, 1977. Fauci, A. S. and Wolff, S. M.: Wegener's granulomatosis and related diseases. DM pp. 1-36, 1977.